Japanese Patent Laying-Open No. 2011-188706 (PTD 1) discloses an uninterruptible power supply device including a plurality of uninterruptible power supply units connected in parallel with one another between an AC power supply and a load. In this uninterruptible power supply device, n (where n is a positive integer) uninterruptible power supply units required to supply a load current are selected from the plurality of uninterruptible power supply units, current is supplied from the selected n uninterruptible power supply units to the load, and operation of each remaining uninterruptible power supply unit is stopped. When the load current increases and cannot be shared by the n uninterruptible power supply units, another uninterruptible power supply unit is activated, and current is supplied from the (n+1) uninterruptible power supply units to the load.